U.S. Pat. No. 2,991,508 discloses the extrusion of smooth-surfaced articles such as film and wire coatings of thermoplastic polymer. As the extrusion rate increases to reach the critical extrusion rate, the surface of the extrudate becomes rough. Upon further increase in extrusion rate, the super shear rate is reached, at which the extrudate surface becomes smooth again. As the extrusion rate is further increased, the extrudate surface becomes rough again, representing the maximum extrusion (shear) rate for the polymer. Usually, the surface roughening just above the critical shear rate has the appearance of sharkskin, i.e. a satin appearance, with the roughness of the surface similar to the texture of very fine sandpaper, barely perceptible to the touch. The severity of the roughness may increase to the point where the surface roughness is clearly visible as surface irregularities and these can be felt by touch. This surface roughness is commonly referred to as gross melt fracture. The surface roughening occuring as the extrusion rate is increased above the super shear rate is gross melt fracture. Thus, it is gross melt fracture that usually forms the limitation on the shear of the polymer and thus its maximum extrusion rate. Some polymers do not exhibit the super shear phenomonon, i.e. the the extrudate surface transforms from being smooth to sharkskin to gross melt fracture, while other polymers transform from smooth surface to gross melt fracture. For many applications, just the sharkskin appearance is objectionable and the extrusion rate at which this occurs forms the maximum extrusion rate.
U.S. Pat. No. 3,125,547 discloses the use of certain fluoropolymers as processing aids in the extrusion of hydrocarbon polymers. This patent was followed by improvements in the fluoropolymer used as the processing aid and combinations of fluoropolymer with certain additives, such as polyethylene glycol and or polar-side-group adjuvants. Generally, the processing aids are effective in postponing the occurence of sharkskin as extrusion rate is increased, but have little effect on the extrusion rate at which gross melt fracture occurs.
Improvement is still needed in the maximum extrusion rate attainable for thermoplastic polymer which is extrusion rate limited by extrudate surface roughness, whether sharkskin or especially, gross melt fracture.